Technical Field
The present invention relates to a semiconductor device and a battery monitoring system, and particularly relates to a semiconductor device and battery monitoring system that detect voltages of battery cells.
Related Art
In general, a battery pack (assembled battery) in which plural batteries (battery cells) are connected in series is used as a high-power battery with high capacity to be utilized for driving a motor in a hybrid automobile or an electric automobile, or the like (as a concrete example, a lithium ion battery or the like can be mentioned). A battery monitoring system that detects and monitors voltages of the battery cells of this battery with voltage detection circuits is known. As an example of this kind of voltage detection circuit, the technology recited in Japanese Patent Application Laid-Open (JP-A) No. 2010-281805 is known.
Another technology is recited in JP-A No. 2001-116776, in which a noise removal filter that removes noise produced by a battery cell is provided between the battery cell and a voltage detection circuit.
An example of a related art battery monitoring system and voltage detection circuit is shown in FIG. 6 and FIG. 7. Here, as a specific example, three battery cells C of a battery cell group 12 are illustrated in FIG. 6. As shown in FIG. 7, a voltage detection circuit 200 of a related art battery monitoring system 100 is provided at each battery cell C.
The voltage detection circuit 200 divides a battery voltage from the battery cell C (a difference between a high potential side voltage and a low potential side voltage of the battery cell) over a resistor Ra and a resistor Rb, and is equipped with a comparison circuit (a comparator 300) that compares the divided voltage with a reference voltage generated by a reference voltage generation circuit, and outputs a comparison result.
If the battery voltages of neighboring battery cells C are different, the related art battery monitoring system 100 as shown in FIG. 6 may not be able to accurately detect the battery voltages of the battery cells C, due to the effect of a noise removal filter 14 connected between the battery cell group 12 and the voltage detection circuits 200.
Operation of the related art voltage detection circuit 200 is described with reference to FIG. 8. As shown in FIG. 8, a current flowing in a battery cell C1 is represented by I1, and an input voltage of the voltage detection circuit 200 thereof is represented by V1. Similarly, a current flowing in a battery cell C2 is represented by I2 and the input voltage of the voltage detection circuit 200 thereof is represented by V2, and a current flowing in a battery cell C3 is represented by I3 and the input voltage of the voltage detection circuit 200 thereof is represented by V3.
The noise removal filter 14 is a low-pass filter (LPF) constituted by an RC circuit. Resistors to which the same reference numeral is assigned in the RC circuits have equal resistance values.
Firstly, a case in which the battery voltages of neighboring battery cells C are equal is described. Specifically, a state in which the battery voltages of the battery cells C (C1, C2 and C3) of the battery cell group 12 are all equal is described.
Of current flowing in a resistor R1 of the noise removal filter 14, a current flowing from a low potential side (lower level) battery cell C and a current flowing from a high potential side (upper level) battery cell C flow in opposite directions. Given that the battery voltages of these battery cells C are equal, the values of the two currents are equal, the currents flowing in the resistor R1 cancel out, and there is no voltage drop across the resistor R1. Therefore, when the battery voltages of the battery cells C are equal, the battery voltage of each battery cell C becomes the input voltage V of the voltage detection circuit 200 thereof, and there are no differences between the input voltages V.
In a resistor R2 that is connected to the highest potential battery cell C3, current flows in only one direction (corresponding to a current flowing from the low potential side battery cell). Therefore, there is a voltage drop across the resistor R2, and there may be a difference between the battery voltage of the highest potential battery cell C3 and the input voltage V of the voltage detection circuit 200 thereof Reducing the resistance value of resistor R2 is commonly used as a measure to moderate the effects of this difference.
Next, a case in which the battery voltages of neighboring battery cells C are not equal is described. As a specific example, a case in which the battery voltage of the battery cell C1 is smaller than those of the other battery cells C (C2 and C3) is described. In this case, the current I1 is not equal to the current I2.
A current that flows in the resistor R1 of the noise removal filter 14 that is connected between the battery cell C1 and the battery cell C2 is the difference between current I1 and current I2. Given that the battery cell C2 has a larger battery voltage than the battery cell C1, current I2 is larger than current I1. Therefore, the current flowing through the resistor R1 is not substantially at zero, and a voltage drop is caused by the current difference between current I1 and current I2. Therefore, there are differences between the input voltages V of the voltage detection circuits 200 and the battery voltages of the battery cells C, and accurate voltage detections may not be possible.